A polymer electrolyte fuel cell has a structure in which a polymer electrolyte membrane is used as an electrolyte and the both sides of the membrane are bound to electrodes.
It is necessary for a polymer electrolyte membrane to have low membrane resistance when used for a fuel cell. For this reason, it is preferable that the membrane thickness be minimized. However, excessive reduction of membrane thickness is liable to result in pinhole formation during membrane making, membrane breakage during electrode making, or short circuits between electrodes, which has been problematic. In addition, whenever polymer electrolyte membranes are used for fuel cells, they are in a moist state. Therefore, moistening causes, for example, swelling or deformation of polymer electrolyte membranes. This causes problems of durability in terms of pressure resistance, cross-leakage, or the like during a differential pressure operation.
Hence, a thin reinforced membrane with a uniform thickness having uniform strength in both the longitudinal and lateral directions has been developed. For example, Patent Document 1 discloses a polymer fuel cell electrolyte membrane comprising a composite for which the tensile yield stress is 12 MPa or more in the longitudinal and lateral directions and the relative value of the tensile yield stress in the longitudinal direction to the tensile yield stress in the lateral direction (tensile yield stress in the longitudinal direction/tensile yield stress in the lateral direction) is 2.0 or less.
Meanwhile, Patent Document 2 discloses, as an ion conductive diaphragm having a high degree of hardness and dimensional stability, a composite diaphragm obtained by allowing an integrated composite diaphragm comprising stretched/expanded polytetrafluoroethylene, which has a morphological structure characterized by a fine structure of nodes with ultra-high extensibility (such nodes being bound to each other via fibrils), to absorb ionomers. It is also disclosed that a composite membrane has extraordinarily improved hardness so as to reduce the occurrence of electric short circuits, thereby improving fuel cell performance and durability.
In general, it has been attempted to form a composite of a porous body such as stretched polytetrafluoroethylene and an electrolyte material so as to reduce the occurrence of electric short circuits, thereby improving performance and durability. However, the porous body structure becomes composite. In order to further improve membrane strength, proton conductivity (specifically, fuel cell performance) must be sacrificed, which is problematic.
Further, a polyelectrolyte material having high proton conductivity and excellent durability has been examined. However, when chemical resistance is imparted to such a membrane, the polymer structure becomes composite. This causes concerns of yield deterioration in the synthesis process and a sharp increase in material cost for synthesis of a novel material or the like. Furthermore, it cannot be said that sufficient polyelectrolyte material strength is achieved in such case. In addition to such problems, a membrane obtained by making a composite of a polytetrafluoroethylene porous body and an electrolyte material has a membrane face with strength anisotropy. Accordingly, such membrane tends to become distorted in fuel cells, facilitating membrane deformation or destruction, which has been problematic.
The above problems have arisen due to lack of simultaneous achievement of improvement of electrolyte membrane strength and provision of chemical resistance. In addition, in order to improve strength based on conventional technology, it is necessary to increase the porous substrate thickness or change the fine porous substrate structure.
Hitherto, porosity has been imparted to polytetrafluoroethylene porous substrates by a stretching method. This often results in a difference between the degree of stretching in the machine direction (for sheet making) (MD) and that in the transverse direction (TD; vertical to the MD direction). Therefore, it has been thought that it would be difficult to change the fine structure or reduce strength anisotropy in the MD and TD.
As an aside, a cell reaction causes generation of peroxide in a catalyst layer formed at the interface between a polymer electrolyte membrane and an electrode in a polymer electrolyte fuel cell. The generated peroxide is dispersed therein, giving rise to the formation of peroxide radicals, and resulting in deterioration of the electrolyte. For instance, oxidation of fuel takes place at the fuel electrode and reduction of oxygen takes place at the oxygen electrode in a fuel cell. The following equations (1) and (2) represent ideal oxidation and reduction reactions, respectively, when hydrogen serving as fuel and an acidic electrolyte are used.Anode(hydrogen electrode):H2→2H++2e−  (1)Cathode(oxygen electrode):2H++2e−+(½)O2→H2O  (2)
Protons generated at the anode as a result of reaction (1) are hydrated to form H+(xH2O) so as to permeate (or to be dispersed in) a polymer electrolyte membrane. The protons that have permeated the membrane are subjected to reaction (2) at the cathode. These electrode reactions at the anode and the cathode take place in a reaction site in an electrode catalyst layer tightly adhering to a polymer electrolyte membrane and progress at the interface between a catalyst and a polymer electrolyte membrane in the electrode catalyst layer.
However, in addition to such main reactions, side reactions take place in actual fuel cells. One representative side reaction causes generation of hydrogen peroxide (H2O2). Although the mechanism of such generation of hydrogen peroxide has not been completely elucidated, the probable mechanism can be explained as follows. Specifically, generation of hydrogen peroxide can take place at either the hydrogen electrode or the oxygen electrode. For instance, it is thought that hydrogen peroxide is generated at the oxygen electrode as a result of an incomplete oxygen reduction reaction represented by the following formula.O2+2H++2e−→2H2O2  (3)
In addition, oxygen contained as an impurity in a gas or mixed with a gas for a specific purpose or oxygen dissolved in an electrolyte at the oxygen electrode and thus dispersed toward the hydrogen electrode is thought to be involved in a reaction at the hydrogen electrode. The reaction formula for such reaction can be the same as formula (3) above, or it can be the following formula.2M−H+O2→2M+H2O2  (4)
Here, “M” represents a catalyst metal used at the hydrogen electrode. “M−H” represents such catalyst metal to which hydrogen has adsorbed. In general, a noble metal such as platinum (Pt) is used as a catalyst metal.
Hydrogen peroxide generated at either electrode is released from the electrode so as to be dispersed, for example, and it is transferred into an electrolyte. Hydrogen peroxide is a substance having a strong oxidizability and therefore it oxidizes many of the organic substances constituting an electrolyte. Details of the oxidation mechanism have not been completely elucidated. However, in many cases, hydrogen peroxide radical formation takes place and the resulting hydrogen peroxide radical directly serves as a reactant in an oxidation reaction. That is, it is thought that a radical generated by the formula given below removes hydrogen from organic substances of an electrolyte or cleaves other bonds. Although the cause of radical formation has not been completely elucidated, it is thought that hydrogen peroxide in contact with heavy metal ions has catalyst activity. In addition, it is thought that heat, light, and the like can cause radical formation.H2O2→2.OHorH2O2→.H+.OOH
As described above, an electrolyte membrane for a fuel cell is required to contribute to the improvement of durability (the reduction of fluorine emissions and the prevention of increases in cross-leakage) and the improvement of output (the prevention of decreases in proton conductivity).
It has been revealed that chemical deterioration of an electrolyte membrane caused by hydrogen peroxide-derived radicals can be inhibited with the addition of Ce (cerium) or a Ce compound to an electrolyte membrane or MEA, leading to the significant improvement of durability.
For instance, Patent Document 3 discloses that a peroxide decomposition catalyst capable of decomposing peroxide is provided to at least one of a pair of electrodes that constitute a membrane-electrode assembly for a polymer electrolyte fuel cell in a manner such that concentration gradient diffusion takes place. This is intended to efficiently decompose peroxide generated in a fuel cell and to inhibit the deterioration of an electrode and an electrolyte membrane.
Specifically, a peroxide decomposition catalyst is provided to an electrode such that concentration gradient diffusion takes place in the electrode. Concentration gradient diffusion may take place in the thickness direction or in the lateral direction of the electrode. Patent Document 3 describes that it is particularly desirable that a peroxide decomposition catalyst be provided to an electrode such that concentration gradient diffusion takes place in the thickness direction of the electrode (paragraph nos. 0021 and 0022).
In addition, Patent Document 4 discloses that a peroxide decomposition catalyst capable of decomposing peroxide is provided to a seal member for sealing the space between an electrolyte membrane and a separator that sandwiches a membrane-electrode assembly from both sides in order to inhibit deterioration of a seal member for sealing the space between an electrolyte membrane and a separator, an electrolyte membrane, and the like so as to improve durability.
Also, it has been revealed that there is a disadvantage to the use of such a peroxide decomposition catalyst capable of decomposing peroxide in that the initial output decreases as a result of the addition of Ce as a peroxide decomposition catalyst. It is thought that such initial output decrease results from the reduction of proton conductivity due to the ion exchange of some sulfonic acid groups in an electrolyte membrane with Ce ions. For instance, it has been revealed that if the amount of Ce added is increased, the initial output decreases to a greater extent. Therefore, it is considered that it is necessary to reduce the amount of Ce added (or use a poorly soluble compound) or to add Ce only to necessary parts of an electrode.    Patent Document 1: JP Patent Publication (Kokai) No. 2004-288495 A    Patent Document 2: JP Patent Publication (Kohyo) No. 2005-520002 A    Patent Document 3: JP Patent Publication (Kokai) No. 2005-235437 A    Patent Document 4: JP Patent Publication (Kokai) No. 2005-267904 A    Patent Document 5: JP Patent Publication (Kokoku) No. 51-18991 A (1976)    Patent Document 6: JP Patent Publication (Kohyo) No. 2006-504848 A    Patent Document 7: U.S. Pat. No. 5,476,589